The `From<Value>` implementation for `Theme` converted the Value to a
string and re-parsed the string to convert it to
`HashMap<String, Value>` which feels a bit wasteful. This change uses
the underlying `toml::map::Map` directly when the value is a table and
warns about the unexpected `Value` shape otherwise.
This is necessary because toml 0.6.0 changes the Display implementation
for Value::Table so that the `to_string` no longer encodes the value as
a Document, just a Value. So the parse of the Value fails to be decoded
as a HashMap.
The behavior for returning `Default::default` matches the previous
code's behavior except that it did not warn when the input Value was
failed to parse.
70887b7378